For the first time in decades, federal rules that govern how and when airline flight crews rest between flights will change Saturday in an effort to limit fatigue and improve safety.

Unlike the old rest rules, which were generally static regardless of a pilot’s schedule, the new ones vary based on data such as the time of day of the first flight, the number of scheduled flight segments and the number of time zones crossed. The new rules were set after officials consulted sleep science research.

Airlines and pilot groups generally support the changes, which were identified as a top priority by the Department of Transportation after a 2009 crash outside of Buffalo, N.Y. — Colgan Air flight 3407 — in which pilot fatigue was identified as an issue. But the changes likely will impact flight operations and cause delays, especially during periods of bad weather when pilots might be in danger of working too many hours.

“We think it could be more of an issue on days that are disruptive,” said Ray Redlich, director of operations management for Delta Air Lines. “They’ve made these hard limits, so there is no wiggle room for crews. That’s more difficult than you would imagine.”

At Delta and other carriers, the biggest issues may occur when flights are diverted. When thunderstorms hit Atlanta — Delta’s biggest hub — pilots may divert planes to other airports throughout the South, like Birmingham, Ala., Nashville, Tenn., and Augusta, Ga. In the past, pilots may have had flexibility to continue the flight to Atlanta. But under the new limits, that could be less likely.

“In the previous rules that governed our flying, you could be scheduled to be at the controls for a certain amount of time, but if you had a weather divert, you could be extended well beyond those limits,” said Capt. Sean Cassidy, first vice president of the Air Line Pilots Association. “These new rules actually put hard stops in the amount of hours that you are going to be at the controls.”

Airlines might have to more often fly a new crew to the diversion city. Or they might choose to send passengers on in buses.

“There’s no way we can have crews placed throughout the system to mitigate that risk,” Delta’s Redlich said.

The new rules have been in the works for two years, so airlines like Delta that have prepared for them and should be ready for most quirks, said Cassidy, whose union represents roughly 50,000 members. The union also has tried to teach its members about the changes, even creating a mobile phone application to help them calculate how much they can work.

Cassidy said one important new rule will ensure pilots have the opportunity for eight hours of uninterrupted sleep and 10 hours of overall rest time between one day and the next. The old guidelines only set an eight-hour minimum overnight break for pilots.

Pilots could always request extra rest time if they felt tired, but Cassidy said that was a difficult question to pose to company officials.

“Even though I was a little bit tired, there was a tremendous amount of pressure put upon me to take the next flight and take everyone home or to where they were going,” Cassidy said, recalling when he was a regular “line” pilot. (In the new system, pilots will still have the ability to turn down work if they feel too tired.)

Robert Mann, an airline consultant based in New York, said passengers might experience more flight delays in the next couple of months, as carriers learn how to best schedule crews. But he said smaller regional carriers — the carriers that use small jets and propeller planes to transport fliers to and from major airline hubs — will be most impacted. They tend to be more aggressive in how they schedule flight crews, he said.

Under the new rules, pilots will be able to be on duty for nine to 14 hours — a number that will fluctuate based on factors such as when the pilot’s day begins and how many segments he or she will fly in one day. Pilots will be able to fly no more than nine hours during that period.

“Honestly, some of these guys have days which can easily run to 16, 17, 18 hours,” Mann said, of regional airlines. “That’s just a long time. It’s a long time if you are not in command of a transport aircraft.”

A spokeswoman for one of the largest regional airlines in the west — St. George, Utah-based Skywest — said in an email: “We expect the customer should experience seamless operations through the transition.”

Other airlines also say they’re ready.

A Southwest Airlines official said the carrier has prepared for the new rules for the past two years and that it has developed new crew scheduling software to ensure pilots follow guidelines. “We are fully prepared to implement the new FAA industrywide rules by the imposed deadline of Jan. 4,” an airline official said in a statement.

JetBlue, an airline official said, has been prepping for 18 months. It also developed an in-house program to track duty time, which pilots will be able to track on their company iPads.

“In the past few weeks of preparation, we’ve run a series of (simulations) for all conceivable scenarios with front-line crew members across the operation to map out how the new rules will impact diversions, complex weather days, our overall operation and our customers,” JetBlue spokeswoman Jenny Dervin said in an email.

At Delta, airline employees actually started following the new guidelines on Wednesday. But even though the airline’s Northeast flights were severely disrupted by a winter storm nicknamed Hercules, an airline spokesman said there were no unusual delays attributable to the new rules.

Redlich, who has been with Delta for 39 years, called the operation a “chess match” but said most of his colleagues have enough experience to handle just about anything that comes their way.

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